Conservation of energy with a mass and pulley system.

  1. 1. The problem statement, all variables and given/known data
    Two boxes are attached to opposite ends of a rope passing over a frictionless pulley as shown below. The mass of Box A is 15kg and the mass of box B is 12kg. The system is originally at rest with the bottom of box A at a height of o.85m above the floor. When the system is released, the boxes will move. Use conservation of energy to determine the speed with which Box A will contact the floor.


    2. Relevant equations

    Eg=mgΔh
    Ek= 1/2 mv^2/2
    ƩFy=may

    3. The attempt at a solution
    I started off by drawing free body diagrams of each mass, one at rest, and one in motion.

    For mass A:
    at rest,
    ƩFy=0
    Ft+Eg=0
    Ft= Eg
    = mgΔh
    =(15kg)(9.8m/s^2)(0.85m)
    Ft=125N

    in motion,
    ƩFy= may
    Fg(A)-Ft= m(A)ay

    For mass B:
    at rest,
    ƩFy=0
    Fn-mg=0
    Fn=mg
    =(12kg)(9.8m/s^2)
    Fn=117.6N

    in motion,
    ƩFy=may
    Ft-Fg(B)= m(B)ay

    I'm not sure if my original statement of Ft=Eg is accurate... and from this point on I don't know where to go.
     
  2. jcsd
  3. Why not find the initial and final energies and equate them?
     
  4. As in, Etotal= Eg, Etotal'=Ek ?
     
  5. System is originally at rest. Hence initial energy = ...
    Final energy = ...
     
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